Band-pass selector



N. P. cAsE BAND-Mss szLzc'lroR Filed Oct, 29, 1 937 QPR NELSON E CAS pBYUMH.. 4 @1Q/4',

`ATTORNEY Patented Oct. 8, 1940 PATENT OFFICE BAND-PASS SELECTOR NelsonP. Case, Great Neck, `N. Y., assigner to Hazeltine Corporation, acorporation of Delal Application ocmber 29, 1937,5seria1N0. 171,641

13 Claims.

This invention relates to band-pass selectors of simplified form and,more particularly, to band-pass selectors having band-passcharacteryistics adjustable about a xed mean frequency. i151 While theinvention is of general application, it

is particularly suitable for use in the intermediate-frequency channelof a superheterodyne radio receiver. i n

It is `frequently `desired to tune a modulated- 10 )carrier signalreceiver to one of two stations on adjacent channels having a signal`strength of the same order as, `or lessthan, that of the adjacentundesired signal. In such a case, it is essential that the receiver haveselector circuits 1'5 which are sufficiently selective to transmit thedesired signalwithout undue interference from .theadjacent undesiredsignal. In general, however, if highly selective circuits are'used, thesideband frequencies remote from the mean freg4) `quency of the` signalchannel are necessarilyattenuated, resulting in a reduction in thequality Vor fidelity of signal reproduction. Such highly selectivecircuits are notlneeded where the desired signal has an amplitudesubstantially g5 greater than that of the signal on `an adjacentchannel.V Under such circumstances, it is desirable that the selecto-rcircuits of the receiver have` va flat-,topped band-pass characteristicwhich is atleast `as broad as the `total modulam tion-frequency band `of`the received signal, so that all `signal ,components are transmitteduniformly.'`

,It is presentpractice, therefore, to provide the receiver with oneormore selector circuits which 35 are adustable `to meet thetwoconditions of receiver operationrdescribed above. h Such selectorcircuits are ygenerally relatively complicated in that they employeither complicated mechanical arrangements to vary the coupling betweentwo or more tunedl circuits, or relatively complicated ,electricalarrangementsfor varying the couplingtherebetween. It has been proposedlnormally to couple the circuits of the selector by means of twocoupling paths having opposite 5 coupling effects, one of which isgreater than the other, thus providing maximum selectivity, and vto varyor remove one of the coupling paths to provide a net coupling greaterthan the critical value and thereby` widen the band of frequencies 50passed.' Circuits of this type previously proposed, A however, havebeen4 such as to detune the selector `circuits in the same directionwhen the couplingbetween the circuitsis varied, thus tending to shift4the mean frequency of the band of 5p `flelllIlies*passed by the,selector.

`It is, therefore, an object of the invention to provide a simplifiedcircuit arrangement for varying the coupling between the tuned circuitsof a band-pass selector and for simultaneously detuning the tunedcircuits symmetrically with respect fr, to the mean frequency of theband.

In accordance with an embodiment of the invention, a band-pass selectorcircuit comprises xed-tuned terminal circuits having a nondirectivecoupling means for coupling the terminal circuits. There is alsoincluded in the. selector circuit auxiliary coupling means coupling theterminal circuits and means for adjusting the width of the pass band ofthe selector circuit comprising means for adjusting the coupling effectof the auxiliary coupling means and for simultaneously eiectivelyintroducing reactive components of opposite sense in the terminalcircuits.

In accordancewith a specific form of the invention, in a band-passselector circuit of the b type described the auxiliary coupling meanshas a coupling effect opposite to that of the first-named couplingmeans, the coupling ,effect between the terminal circuits being lessthan the critical value of coupling therebetween, and the means foradjusting the width of the pass b-and of the selector circuit comprisesmeans for decreasing the coupling effect of the auxiliary coupling meansand for simultaneously detuning the terminal circuits in oppositesenses, the reactive constants of the coupling circuits being such thatthe effective gain of the selector circuit remains substantiallyconstant.

The novel features which are believed to be characteristic of thisinvention are set forth with particularity in the appended claims. Theinvention itself, however, both as to its organization and method ofoperation, together with further objects and advantages thereof, willbest be understood by reference to the following specification taken inconnection with the accompanying drawing, in which Fig. l is a circuitdiagram, partly schematic, lof a radio broadcast receiver of thesuperheterodyne type embodying a band-pass selector constructed inaccordance with the invention, while Figs, 2 and 3 represent differentembodiments of the band-pass selector of Fig. 1, per se.

Referring now more particularly to Fig. 1 of the drawing, there isillustrated one form of the improved band-pass selector system embodiedin v a radio broadcast receiver of the superheterodyne type,` forcontrolling the selectivity of the intermediate-frequency channel of thereceiver. Briefly described, the receiver comp-rises a radiofrequencyampliiier I9 connected to an antennaground circuit II and coupled to anoscillatormodulator or frequency changer I2. Connected in cascade withthe output circuit of the frequency changer I2 are anintermediate-frequency amplifier I3, a band-pass selector I4,intermediate-frequency amplifier I5, a detector and automaticamplification control or A. V. C. supply I6,

, an audio-frequency amplifier II, and a sound reproducer I8. A biaspotential derived from the automatic amplification control source I6 maybe applied, by way of conductor I6', to one or more of the stages of theradio-frequency amplifier I0, to the oscillator-modulator I2, and to oneor more of the stages of each of intermediate-frequency amplifiers I3and I5.

Considering first the operation of the receiver as a whole, withoutregard to the details of the band-pass selector I4, per se, the desiredreceived signal wave is selected and amplified in the radiofrequencyamplifier I0 and is converted into a modulated intermediate-frequencycarrier by the frequency changer I2. The signal as thus converted isfurther amplified in intermediate-frequency amplifier I3, selected andamplified in selector circuit I4 and intermediate-frequency amplier I5,and rectied in the detector I 6, thereby producing the audio frequenciesof modulation, which are, in turn, amplified in the audio-frequencyamplier II and reproduced by the sound reproducer I8. The amplication ofthe received signal is subject to automatic amplication control by thecontrol-bias potential derived from the source I6, according to themanner well understood in the art.

Referring now more particularly to the details of the band-pass selectorsystem, indicated generally at I4, and constructed in accordance withthe invention, the system comprises a fixed-tuned input or terminalcircuit I9 and a fixed-tuned output or terminal circuit 20, whichcircuits are coupled to the output circuit of intermediatefrequencyamplifier I3 and to the input circuit of intermediate-frequencyamplifier I5, respectively. In order to provide a nondirective couplingbetween the terminal-circuits, the inductance elements of the circuitsI9 and 20 are inductively coupled, as indicated by the bracket M. Forproviding a coupling eiect between the terminal circuits I9 andZopposite to that provided by the coupling M, there is provided anauxiliary coupling means in the form of a link circuit comprisingserially-connected inductances 2I and 22 individually coupled to theinductance elements of the circuits I9 and 20, respectively. The mutualinductance between the inductance 2| and the circuit I9 is` representedby the bracket M1, while the corresponding mutual inductance betweeninductance 22 and circuit 29 is represented by M2. One side of the linkcircuit is grounded at 23, while a switch 24, having contacts 25 and 2E,is provided in the other side of the link circuit in order to open theseries connection of inductances 2I and 22 and short-circuit inductance2I. The circuit constants of the arrangement just described are soadjusted that the coupling effect due to the mutual inductance M isconsiderably greater than that of the link circuit between the tunedterminal circuits I9 and 20.

In considering the operation of the selector just described, it will beassumed that, with the switch 24 closed at 25 so that the couplingeffect of the link circuit opposes that of the mutual inductance M, thecircuit constants are such that the resultant coupling between thecircuits I9 and 20 is critical or less than critical, so that theselector has relatively great selectivity. When the switch 24 is openedat 25 and closed at 26, the opposing coupling effect of the link circuitbetween tlie tuned terminal circuits I9 and 20 is eliminated, while atthe same time the effect is to remove inductance 22 from the selectorcircuit and to short-circuit the inductance 2 I. Under this condition,the resultant coupling between the circuits I9 and 2|J is materiallyincreased and the pass band of the selector correspondingly widened. Atthe same time, the effective inductance in circuit I9 is materiallyreduced, because of the fact that the terminals of the low-inductancewinding 2| are short-circuited rather than being connected through theimpedance of inductance 22, and the inductance 22 is effectually removedfrom the selector circuit and, therefore, the inductance reflected by itacross the circuit 29 is practically eliminated. As a result, circuitsI9 and 29 are detuned in opposite directions from the mean resonantfrequency of the intermediate-frequency channel of the receiver. Thisalso has the effect of widening the band of frequencies passed byselector I4, without shifting the mean resonant frequency of the band,that is, without causing the band-pass characteristicl of the selectorto become unsymmetrical about the mean frequency of the band. The switch24 and its connections with the windings 2I and 22 of the link circuitthus constitute means for adjusting the width of the pass band of theselector circuit, comprising means for adjusting the coupling eiect ofthe auxiliary coupling means and for simultaneously eiectivelyintroducing reactive components of opposite sense in the terminalcircuits.

In another embodiment of the invention, the link circuit may take theform shown in Fig. 2, wherein parts which are identical with the linkcircuit of Fig. 1 are given similar reference numerals. In theembodiment shown in Fig. 2, the switch 24 has been omitted andinductance 22a comprises an adjustable tap 28 connected in the linkcircuit, as shown. It will readily be seen that, at one limitingposition of the tap 28 of Fig. 2, the inductance 22a is entirelyincluded in the link circuit and, at the other limiting position, theinductance 22a is eectively removed from the link circuit and theinductance 2| is short-circuited. Thus, the arrangement of Fig. 2provides, in addition to the advantages of Fig. 1, ,a further, advantagein that the selectivity of the band-pass selector may be graduallyvaried between the two limiting values.

In a further embodiment of the invention, the link -circuit may be asshown in Fig. 3 and comprise a resistor 29 connected across inductance22 and provided with an adustable tap 30. The operation of the circuitof Fig. 3 is similar to that of Fig. 2 and no further explanation isdeemed to be necessary.

It will be understood that, if the circuits I9 and 20 are criticallycoupled by means of the mutual inductance M and the link circuitcomprising inductances 2l and 22, the effect of symmetrically detuningthe terminal circuits I9 and.' 29 by means of the operation of theswitch 24 or adjustable contacts 28 o-r 30 is materially to decrease thegain of the selector. It will further be understood that the selector,I4 may be so designed that the coupling eiects of the mutual inductanceM and the link circuit comprising the inductances 2I and 22 result in anet Iii) . the selector lil.

coupling which; isconsiderably less than critical. In this case, theeffect of operatingswitch 24 or adjustable contacts 28` or 30 toincrease the coupling tbetween terminal circuits I9 `and 20 is. toincrease thegain of the selector system, whilethe effect ofsymmetrically detuning the terminal circuits-is to decreasel the gainacross Thus,`the system can be so designed that these effects arecompensating and Ia substantially constant gain is obtained in eachposition. ofswitch 24 or contacts'28 or 30. It will be furtherunderstood that in another-fem- Ibodiment of the invention the terminalcircuits lil and 2D `may initially be symmetrically detuned when thelink circuit is completed between circuits I9 and 20 and that the`linkcircuit may be adjusted eiectively to reflect reactance components intothe terminal circuits to tune them to the same frequency and at the sametime` decrease the totalcoupling between the terminal circuits. .n f

While there have been described whatv are at present considered to bethe preferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed in the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

What is claimed is:

l. A band-pass selector circuit comprising `xed tuned terminal circuitshaving a nondirective coupling means therebetween, auxiliary couplingmeans for coupling said terminal circuits, and means for adjusting thewidth of the pass band of said selector circuit comprising means foradjusting the coupling eifect of said auxiliary coupling means and forsimultaneously effectively introducing reactive components of oppositesense in said terminal circuits.

2. A band-pass selector circuit comprising fixed tuned terminal circuitshaving a nondirective coupling therebetween greater` than the criticalvalue, auxiliary meansfor coupling said terminal circuits having acoupling effect opposite tof that of said first-named coupling, andmeans for adjusting the width of the pass band of said selector circuitcomprising means for decreasing the coupling effect of said auxiliarycoupling means and for simultaneously effectively introducing reactivecomponents of opposite vsense in said terminal circuits.

3. A band-pass selector circuit comprising fixed tuned terminal circuitsnormally resonant at the same frequency and having a nondirectivecoupling mean-s therebetween, auxiliary coupling means for coupling saidterminal circuits,

`and means for adjusting the width of the pass band of said selectorcircuit comprising means for adjusting the coupling effect of saidauxiliary coupling means and for simultaneously introducing reactivecomponents of opposite sense in said terminal circuits.

4. A band-pass selector circuit comprising fixed tuned terminal circuitsnormally resonant at the same frequency and having a nondirectivecoupling therebetween, auxiliary coupling means for coupling saidterminal circuits having a coupling .effect opposite to that of saidiirst-named coupling, and means for adjusting the Width of the pass bandof said selector circuit comprising means for decreasing the couplingeffect of said auxiliary coupling means andfor simultaneouslyeffectively detuning said terminal'circuits in opposite senses.

5. Awband-pass selector circuit comprising iixed tuned terminal circuitsnormally resonant at different frequencies and having a nondirectivecoupling therebetween, auxiliary means for coupling said terminalcircuits having a coupling effect'of the same se-nse of that o-f saidfirstnamed coupling, and means for adjusting the width of Vthe pass bandof said selector comprising means for decreasing the coupling effect ofsaid auxiliary means and forr simultaneously* effectively tuning saidterminal circuitsv toward for simultaneously ldetuning said terminalcircuits in opposite senses, the reactive constants of said couplingcircuits being such that the efi fective gain of said selector remainssubstantially constant.

7. A band-pass selector circuit comprising fixed tuned terminal circuitshavingr a nondirective coupling therebetween, auxiliary means comprisingan aperiodic link circuit for coupling said terminal circuits, and meansfor adjusting the width of the pass bandv of said selector circuitcomprising means for adjusting the coupling effect of said link circuitand for simultaneously effectively introducing reactive components ofopposite sense in said terminal circuits.

8. A band-pass selector circuit comprising fixed tuned terminal circuitshaving a nondirective coupling therebetween, a link circuit comprisingtwo inductances individually inductively coupled to said terminalcircuits, said link circuit having a coupling effect in opposite senseto that `of said first-named coupling, and means for adjusting the widthof the pass band of said selector circuit comprising means fordecreasing the -coupling effect of said link circuit and forsimultaneously andoppositely adjusting the reactive effects which saidinductances reiiect into their respective coupled circuits. i

9. A band-pass selector -circuit comprising fixed tuned terminalcircuits having a nondirective coupling therebetween, a link circuitcoupling said terminal circuits comprising two inductances individuallyinductively coupled to said terminal circuits, the coupling eifect ofsaid link cir-cuit being less than and opposite to that of saidfirstnamed coupling, and means for adjusting the Width of the pass bandof said selector circuit comprising means for gradually adjusting thecoupling effect of said link circuit and for simultaneously graduallyadjusting oppositely the reactive effects of said inductances upon theirrespective coupled cir-cuits.

10. A band-pass selector circuit comprising xed tuned terminal circuitshaving a nondirective coupling therebetween, a link circuit comprisingtwo inductances individually inductively coupled'to said terminalcircuits, the coupling eifect of said link circuitbeing opposite to andless than that of said first-named coupling, and

means for adjusting the width of the pass band of said selector circuitcomprising means to adjust one of said inductances to vary the couplingeffect of said link circuit and simultaneously oppositely to vary thereactive effects of said inductances upon their respective -coupledcircuits.

11. A band-pass selector circuit comprising fixed tuned terminalcircuits having a nondirective coupling therebetween, a link circuitcoupling said terminal circuits comprisin-g two inductances individuallyinductively coupled to said terminal circuits, the coupling effect ofsaid link circuit being opposite to and less than that of 'saidfirst-named coupling, and means for adjusting the width of the pass bandof said selector circuit comprising a resistor coupled across one `ofsaid inductances and having an adjustable portion connected across sai-dlink circuit for adjusting the coupling effect of said link circuit andfor simultaneously oppositely varying the reactive effects of saidinductances upon -their respective coupled circuits.

12. A band-pass selector circuit comprising, xed-tuned resonant terminalcircuits having a nondirective coupling therebetween, a link circuitcomprising two reactances individually coupled to said terminalcircuits, said link circuit also providing a coupling effect betweensaid terminal circuits, and means for adjusting the width of the passband of said selector circuit comprising means for changing the couplingeiect of said link circuit and for simultaneously and oppositelyadjusting the reactive effects which said reactances insert in theirrespective coupled circuits.

13. A band-pass selector circuit comprising, fixed-tuned resonantterminal circuits having a nondirective coupling therebetween, a linkcircuit comprising two reactances individually coupled to said terminalcircuits, said link circuit also having a coupling effect between saidterminal circuits, and means for adjusting the Width of the pass band ofsaid selector circuit comprising means for opening said link circuit andfor simultaneously shorting one of said reactances, whereby the reactiveeffects which said reactances insert into their respective coupledcircuits are changed equally and oppositely.

NELSON P. CASE.

